It Came from Outer Space?

In 1981, I was a beginning graduate student taking a course in field
geology at UC Berkeley. This was only a year after Berkeley
physicist Luis Alvarez, his geologist son Walter and two colleagues
published what was then a startling (and not-much-believed) theory
suggesting that the impact of an asteroid or comet caused, among
other things, the extinction of the dinosaurs. If these Berkeley
luminaries could offer up such patently absurd ideas, the students
figured we were entitled to do the same. So whenever our professor
queried us about some puzzling geologic structure, we had a ready
response: "Must have been an asteroid." Alvarez's theory
ultimately triumphed, and appreciation of the importance of impact
events grew enormously within the geological community. Now earth
scientists are far more ready to accept the validity of
extraterrestrial influences. But a recent episode suggests that the
pendulum might have swung too far.

In 2002, Jens Ormö, Angelo P. Rossi and Goro Komatsu, working
at the International Research School of Planetary Sciences in
Pescara, Italy, reported evidence for what they claimed was a
relatively recent meteorite strike: a field of craters located in
the Abruzzi Apennines, roughly 100 kilometers east of Rome. The
largest feature of the field is a 100-meter-diameter circular basin,
situated in the Prato del Sirente plain, close to the town of
Secinaro. Associated with the main basin are 17 nearly circular
depressions, which presumably formed at the same time because the
extraterrestrial object responsible for them broke up in the
atmosphere just before hitting.

Ormö's team was unable to locate any definitive markers of an
impact, such as meteoritic material emplaced below a crater or
telltale grains of shocked quartz in the target rock. But these
signs could be missing for good reason: Quartz is almost absent from
the limestone-rich sediments found in the area, and perhaps the
group's 4.5-meter-deep excavation of one of the craters had been too
shallow to reach the meteorite they believed to be buried below.
Ormö and his colleagues did find one line of evidence that they
found very compelling—curious magnetic anomalies associated
with many of the smaller craters, which they interpreted to mean
that remnants of meteorites (which are quite often highly magnetic)
were indeed buried there.

In 2003, Ormö and his two coworkers, joined by Roberto
Santilli, used radiocarbon dating to argue that the meteorite that
formed this crater field might have done more than just that,
publishing their ideas in the journal Antiquity. Their
finding that the impact took place in the 4th or 5th century A.D.
fit well with a locally preserved legend that describes people
seeing a star falling to earth, an event that was seemingly
important to their conversion from paganism to Christianity. These
authors also proposed ties with the conversion of Emperor
Constantine himself, which took place at very roughly the same time
and place and was said to have been preceded by notable celestial phenomena.

Not surprisingly, this intriguing story garnered the attention of
the popular press. For example, last year New Scientist
published a piece entitled "Crater find backs falling star
legend." It seems the glib answer I gave to my geology
professor a quarter-century ago had become mainstream.

My graduate student career was not long enough to see this shift in
the attitude of the scientific community through, but it was long
enough to introduce me to Pierre Rochette, a French rock magnetist
who later became a close friend. So I was quite interested to learn
that earlier this year he and two Italian colleagues, Fabio Speranza
and Leonardo Sagnotti, published a challenge to the notion that the
circular depressions on the Sirente plain are impact craters at all,
much less ones that have anything to do with Constantine's
conversion to Christianity. (I should note that, having personal
connections with one of the players in this debate, I harbored some
bias toward his position from the outset.)

Rochette, who normally works out of the University of Aix-Marseille,
became interested in the topic while on a sabbatical at the Istituto
Nazionale di Geofisica e Volcanologia in Rome, where he discovered
that one of his new colleagues was very skeptical of the crater
theory. Speranza, a structural geologist, explains the source of his
initial doubt: "I have a house about 10 kilometers away. I've
known this place since I was a child," adding, "I knew
that the landscape of Abruzzi was full of similar
shapes." Could they all be impact craters? Surely not, he thought.

Speranza points out another difficulty with the impact-origins
theory. Large blocks of limestone sit within the boundaries of the
Sirente "crater." Such limestone would not have survived
an impact. So if Ormö's theory is correct, one must surmise
that somebody set these giant chunks of rock in place since the
crater formed. To Speranza, that just didn't make sense. Speranza
and colleagues further argue that Ormö's radiocarbon dating
gave one age for the main feature (placing it in the 4th or 5th
century a.d.) and a completely different age for a nearby
"crater" called C9, a date in the 3rd millennium B.C.

Indeed, to Speranza, the only suggestive evidence for an impact
origin seemed to be the magnetic anomalies that Ormö and his
colleagues had measured over some of the smaller depressions. But
according to Rochette, even those anomalies are easy enough to
understand. One needs simply to realize that these pockets are
"dolines," places where the limestone has dissolved and
the hole has filled in with sediments that are slightly more
magnetic. Careful measurements of the magnetic properties of these
materials showed that this mechanism is sufficient to account for
the magnetic anomalies.

If not an impact crater, what is the large circular depression found
in the Sirente plain? Speranza, Sagnotti and Rochette give a
plausible answer: It is a reservoir made by human hands for the
purpose of watering herds of sheep. They describe how this area of
Italy was one of the main wool-producing regions of Europe between
the 12th and 16th centuries, although locals have been involved in
the activity since before Roman times. The great permeability of the
underlying rock, however, does not allow rivers or even large
springs to form, which creates a problem for shepherds trying to
maintain millions of sheep there through the summer. The logical
answer to this problem, they posit, was to dig reservoirs at the low
points of these plains, where water tends to accumulate.

Ormö, Rossi and Komatsu have refused to be questioned about
this recent challenge to their theory. In a written reply to my
request for an interview, Ormö states: "It is not possible
for us to comment in [the] media on the work done by other
scientists and on our own unpublished results." Fair enough.
Curiously, the short written remarks these authors shared with me
appear far more tentative than the statements given in their
published papers. They say: "As long as the structure is not a
proven impact crater field, it is impossible to draw any conclusions
about historical consequences." This tone is in stark contrast
to almost the entire body of their Antiquity paper, which
is all about linking the structures seen in the Sirente plain with
historical events.

Speranza notes that officials in the nearby town of Secarino are now
in a bit of a quandary. After Ormö's papers were published,
they began promoting the site as a crater park, hoping to make it a
local tourist attraction. In August of last year they held a meeting
on the "crater," which brought together many of the local
dignitaries. But now that a significant scientific challenge has
been published, it is hard to see how officials of the community of
Secarino can in good conscience go ahead with those plans. After
all, what tourist would want to visit a "Crater or
Just-Plain-Watering-Hole Park"?—David Schneider